1. Field of the Invention
The present invention relates to a method for measuring the quantity of light emergent from a light shutter array made of PLZT or emitted from an LED array and an image forming apparatus which forms an image (electrostatic latent image) on a recording medium with such an optical tip array.
2. Description of Related Art
Conventionally, various kinds of optical write heads which turn on and off light for each pixel with use of a light shutter array made of PLZT or an LED array have been used to form images (electrostatic latent images) on a silver-salt print sheet or film or an electrophotographic photosensitive member. For formation of images without unevenness, such an optical write head needs to be subjected to measurement of the quantity of light outputted from each element and to correction in quantity of light according to the measurement result.
Japanese Patent Laid Open Publication No. 61-150286 has disclosed a method of measuring the quantity of light outputted from each element. According to this publication, while LEDs are turned on one by one in order in a main scanning direction, a light-quantity sensor which faces the LEDs at a specified distance therefrom is moved in the main scanning direction. In such a one-by-one lighting method, it is necessary to synchronize the turning-on/off of the LEDs with the movement of the sensor accurately, and minute control of the start position of the sensor is necessary. Accordingly, an encoder and an accurate shifting mechanism need to be provided. Also, from the result of measurement in this method, influence of light leaking from adjacent elements cannot be recognized, and in forming a thick image, a problem of unevenness is left unsolved.
Further, for formation of a multi-tone image, it is necessary to measure the output light-quantity characteristic of each element accurately. It is insufficient to measure the quantity of light outputted from each element only when it is on as disclosed by the method of Japanese Patent Laid Open Publication No. 61-150286.
Also, conventionally, an optical write head is fitted in a jig before it is employed in an image forming apparatus, and the quantity of light outputted from each element is measured with a light-quantity measuring device comprising a photosensor. Then, correction data are produced based on the results of the measurement. However, these correction data are effective only in the early time of the optical write head and are not useful when the output characteristic of each light element changes because of aging and/or a change in environmental conditions.
Meanwhile, Japanese Patent Laid Open Publication No. 6-347925 has suggested an image forming apparatus which is provided with three optical write heads so as to form an image at a high speed. For this type of apparatus, it is more important to measure the quantity of light outputted from each element and carry out correction at real time. However, this is left unachieved.